Cooling tower having reduced microbial contamination

ABSTRACT

The invention relates to a cooling tower in which contamination with and proliferation of microorganisms can be prevented by incorporating composite materials and/or material combinations and an antimicrobial substance containing tungsten and/or molybdenum into the cooling tower.

The invention relates to improved cooling tower installations, in which the contamination with microorganisms can be prevented. Further, the invention relates to the improvement of the efficiency of the cooling towers by extended constructive scope due to non-present fouling.

In many large-scale industrial processes, however in particular in power plants, a portion of the heat generated therein has to be immediately dissipated again to the environment. This is also referred to as the so-called waste heat.

The simplest type of heat dissipation is the fresh water flow cooling. In this method, the waste heat is directly introduced into waters like rivers and lakes.

However, at industrial and power plant locations, this method is rarely applied, since in the most cases, the allowed heating of the rivers and lakes is already reached. There and at locations, at which waters rich in water are not available, the waste heat has to be immediately released to the atmosphere. This is effected with the aid of cooling towers, which can be operated as so-called “natural draft” or “ventilator cooling towers”. In the ca. 100-170 m high “chimney” of a natural draft cooling tower, the heated air sucks fresh cooling air from the environment into the cooling tower solely by its lift, whereas the air flow is generated in the only about 40 m high ventilator cooling towers mainly by sucking or pressing ventilators.

In cooling towers, one generally differentiates between three variants, wherein all of them can be operated either as natural draft or ventilator cooling towers and have a vertically disposed cooling tower shell. The three variants are dry, wet and hybrid cooling towers.

In dry cooling towers, the heat is released to the air by convection through heat exchangers. For supporting the convection, the heat exchangers include cooling fins. This effect is intensified if the dry cooling towers can be operated as ventilator cooling towers.

Wet cooling towers use water for cooling, which is extracted from the surrounding waters. In order to dissipate the waste heat, the cooling water heated in a condenser is sprayed. Subsequently, it flows down on flow plates in immediate contact with the air, drops from the lower edges thereof and is collected in the cooling tower reservoir, also referred to as cooling tower basin.

In this procedure, approximately ⅔ of the power plant heat is dissipated by evaporation of small amounts of cooling water and ⅓ is dissipated by heating the passing air. The humidification and heating of the air result in a decrease of the density and thus in an increase of the lift of the air. Above the cooling tower, the mixture becomes visible as clouds of steam.

There are again two types of how such a wet cooling tower can be operated. In the so-called “cycle operation”, the recooled water flow is returned to the condenser, in the so-called “drain operation” it is introduced into the water, from which it was extracted.

In hybrid cooling towers, the technical-physical advantages of dry cooling towers (devoid of clouds) and wet cooling towers (high cooling power, better efficiency) combine. However, with respect to wet cooling towers, they have an inferior efficiency due to the power requirement for the necessary ventilators. In addition, the investments for hybrid cooling towers of the same power are very much higher. Thus, more often, preference is given to the wet cooling towers.

A problem arising in operation of cooling towers, in particular wet cooling towers, is the contamination with microorganisms, also referred to as fouling. The undesired deposition and proliferation of the microorganisms in a system or the environment thereof is understood by this term.

Microorganisms such as bacteria and fungi are omnipresent in our biosphere and colonize surfaces of different kind. Many microorganisms are pathogens and their propagation and control therefore play a great role, respectively, since microorganisms can cause life-threatening infections.

Thus, in the used cooling water too, there are microorganisms entering the cooling tower, in particular the cooling system. Temperatures of 30 to 40° C. exist in cooling towers, these and the moisture conditions existing there promote the microbial growth in the cooling towers. Thus, a great proliferation of the microorganisms located and deposited in the system can occur in cooling towers. By the exit of the air or of the cooling water, the microorganisms enter the environment and thus can present an environmental risk. This is in particular the case in wet cooling towers, if the used cooling water already contains a high amount of microorganisms and nutrients, the moisture conditions within the cooling tower are optimum and the water thus contaminated to a high degree immediately enters the courses of rivers, which results in large-area propagation of the microorganisms.

In addition, the depositing microorganisms often form slime, which can result in obstruction of the installations present in this system, such as pipes, flow plates, filters, droplet separators and sieves etc., or narrow clearances in installations are not possible although they would improve the thermal benefit.

A detailed representation of the problems of the contamination of cooling towers with microorganisms, in particular in wet cooling towers, is found in the research report of VGB Technische Vereinigung der Groβkraftswerksbetreiber “Mikrobielle Emission and Immission sowie Keimzahländerungen im Kühlwasser beim Betrieb von Nasskühltürmen”, edited by the VGB Forschungsstiftung, 1979, J. Borneff, G. Ernst, H. P. Werner and D. Wurz.

In the past years, very different measures have been taken in order to prevent the contamination of the cooling towers with microorganisms.

Thus, it has been attempted to construct cooling tower systems, in particular flow plates, such that the deposition of microorganisms is impeded and prevented in optimum case. However, these approaches were mostly not successful, and a non-insignificant bacterial growth within the cooling tower and thus a non-insignificant contamination of the environment continued to be able to be determined. This entails that it is required to clean the cooling towers despite of the particular construction of the flow plates. However, this implies that the cooling system and the corresponding installations are accessible, i.e. they must not be packed too closely within the cooling tower. Furthermore, additional installations/constructions have to be present within the cooling tower, which allow the access to the installations to be cleaned. However, these additional installations result in the fact that further deposition surfaces for microorganisms are provided on the one hand; on the other hand, they prevent optimum uniform spraying of the cooling water or optimum uniform cooling airflow as it would be desirable in cooling towers. All of these required measures result in the efficiency of the cooling towers being degraded.

In wet cooling towers and in hybrid cooling towers, there is the possibility of mixing the used cooling water with antimicrobial substances, also referred to as biocides. The employment of hypochlorous or hypobromous acid, such as described in WO 90/15780, is conceivable herein. However, other biocides can also be used, here, only 3-isotiazol microbiocide is mentioned. However, in particular this biocide has the disadvantage that it only has poor storage stability. Therefore, the employment of suitable stabilizers becomes necessary, such as described in EP 09 109 51.

However, the employment of biocides and stabilizers results in the fact that they and their degradation products get into the cooling water and from there into the recipient, but also into the air, which in turn implies great environmental problems, since biocides not yet used, but also the degradation products thereof, also kill microorganisms, which are responsible for the ecological balance of the waters.

Therefore, it would be desirable to provide cooling towers, in which the employment of biocides is not necessary.

In particular in wet cooling towers, the droplet separators and flow installations located therein, which are constituted of pipelines, nozzles, spraying plates, flow plates and a support construction, are particularly affected by the contamination with microorganisms.

One possibility could be in painting or coating these installations with substances having antimicrobial effect. This possibility is well known in ships or other easily accessible systems, such as described in WO 96/038508 or WO 00/50521, but does not present a really good alternative in cooling towers, since such coating as it is performed in ships, would cause too high cost within the scope of cooling towers. By the construction of the cooling towers, the cooling systems and the installations thereof are only very difficultly accessible. The technical effort, which would have to be made for such coating, would be unprofitable.

The employment of nanosilver, as it is often used within the scope of the health and hygienic care, would also cause high cost. This is due to the high silver price on the one hand, on the other hand, the silver has to be processed to nanoparticles, which is very expensive and costly. A further problem is in that nanosilver tends to form agglomerates, aggregate or clusters in processing. Thereby, the active surface is reduced and in further consequence also the antimicrobial effect of the nanosilver. In order to prevent this, nanosilver is deposited on particle surfaces of a carrier, for example titanium dioxide, which in turn increases the manufacturing cost. Thus, the use of nanosilver in cooling towers and in the installations located therein is also eliminated altogether.

A further possibility of preventing the microbial attack in cooling towers is described in EP 0 544 502. There, it is proposed to use copper oxide and/or zinc oxide in the form of mixed oxides, which can then be incorporated in corresponding plastic articles.

It is known that copper and zinc ions have antimicrobial effect. Due to their inexpensiveness, therefore, they are often used in the control of microorganisms.

However, as it has become apparent and is described in detail in EP 0 544 502, the incorporation of inorganic compounds of copper and/or zinc in plastics results in the fact that the weather and heat resistance of the corresponding products, in particular in resins or rubber, is degraded. In addition, it is known that the employment of these compounds can result in decomposition and foam formation of resins and rubbers at the processing temperature of the resins and rubber, which results in inferior processibility of the plastic and in destabilization of the plastic product.

A further problem is that the used antimicrobial substances do no longer exhibit antimicrobial effect after processing into the corresponding plastics or at least their effect is greatly reduced. The cause for this is that the concentration of the metal ions inducing the actual antimicrobial effect hardly enters the systems to be treated, e.g. cooling water, to the extent that they are able to take their effect.

For these reasons, in EP 0 544 502 it is proposed to use copper and zinc in the form of mixed metal hydroxides. The proposed mixed metal hydroxides are a solid solution composed of Cu²⁺ and/or Zn²⁺ as antimicrobial component in connection with Ca(OH)₂ and Mg(OH)₂. These mixed metal hydroxides allow that sufficient copper and/or zinc ions are released into the water due to the improved solubility, and thus antimicrobial effect can be achieved.

However, the disadvantage of this system is that the antimicrobial agent has to be dissolved out of the plastic in order to take its effect, and therein is consequently consumed. This results in the fact that the plants regularly have to be checked for their still present antimicrobial protection. Furthermore, installations have to be renewed merely due to this no longer present protection, which in turn causes additional cost.

Furthermore, there exists the risk that metal ions not “consumed” or undesired byproducts such as copper sulfate enter the waters via the cooling water and there kill microorganisms, which are vital to the ecological balance of the waters.

Thus, it was the object of the present invention to provide cooling towers with installations, in which germ fouling is prevented, wherein the antimicrobial active agent is to have a good cost-benefit ratio, is nearly not consumed in order to be able to exclude extensive service and maintenance measures, and does not present any environmental risk.

In addition, an additional cleaning of the installations should no longer be required on order that corresponding installations and constructions provided thereto can be omitted and thus the efficiency of the cooling tower can be optimally exploited.

Furthermore, the antimicrobial active agent should be able to be incorporated in the cooling towers, in particular in the installations present therein, in order to avoid complicated and expensive painting or coating works, wherein the substance must not lose its antimicrobial effect and it is not decreased, respectively, by the incorporation. In addition, the mechanical resistance must not be degraded by the antimicrobial active agent.

This manifold spectrum of objects is solved in the present invention in that a cooling tower is provided, which contains a cooling system, which has at least one installation of at least one composite material and/or at least one material combination and a substance causing the formation of hydrogen cations in contact with an aqueous medium and containing molybdenum and/or tungsten.

It has become apparent that under the conditions existing in cooling towers, in particular the temperature and moisture conditions, the efficacy of substances containing molybdenum and/or tungsten with antimicrobial effect is greatly increased.

This effect was not expected in this manner, since with known antimicrobial active substances, a degradation of the antimicrobial effect could always be observed, if they are incorporated in installations or products and tested under cooling tower conditions. At best, the antimicrobial efficacy of the substance remained unchanged.

In particular, it could be demonstrated in test series that in comparison with systems containing copper, samples containing molybdenum and/or tungsten have a 100 times greater antimicrobial efficacy under the conditions existing in cooling towers. In addition, it could be proved that these substances are nearly not consumed in contrast to e.g. substances containing copper, and thus a temporally unlimited use of the substances is possible.

Furthermore, it could also be demonstrated that formation of undesired molybdenum and/or tungsten byproducts does not occur, which could then enter the waste waters via the cooling water, as it is the case with copper tending to the formation of copper sulfate (CuSO₄).

In addition, it has become apparent that the stability and weather resistance of the installations is not impaired neither by tungsten nor molybdenum or the compounds thereof or a combination of both.

The cooling system can be an air or cooling water system or a combination thereof. A cooling water system is preferred.

At least one installation of the cooling tower according to the invention contains the antimicrobial active substance.

As already mentioned, the risk of contamination with and proliferation of microorganisms is particularly great in installations such as droplet separators and flow installations in wet cooling towers. Therefore, it is preferred that the droplet separator and/or the flow installation, as it is usually used in wet cooling towers, contains the antimicrobial active substance.

Usually, flow installations are constituted of nozzles, spraying plates, plural flow plates and a support construction. In the present invention, it is preferred that at least one component of the flow installation contains the antimicrobial active substance, it is particularly preferred when the component is a spraying plate and/or flow plates.

Herein, the spraying plate is preferably made of plural grid-shaped plates, which are produced in injection molding procedure and are arranged in a layer such that they form the plate.

The flow installations include plural flow plates disposed side by side.

Flow plates, as they are usually used in wet cooling towers, in turn are constituted of plural layers, preferably of 5 to 20 layers, particularly preferred of 8 to 15 layers.

Usually, each layer has a layer thickness of 1.00 to 2.5 mm, a layer thickness of 1.5 to 2.0 mm is preferred.

Each layer is formed of fibers, which are woven to each other in grid-like manner. The channels of the grid-like layer have a preferred diameter of 10 to 30 mm, further, a diameter of 15 to 25 mm is preferred.

The layers are fused together or adhered and thus constitute the flow installations.

The installations of the cooling tower according to the invention are produced from composite materials and/or material combinations.

By a composite material, a material of two or more connected materials is understood. The composite material has other material characteristics than its individual components. For the characteristics of the composite materials, material characteristics and geometry of the components are of importance. In particular, size effects often play a role. The connection is effected by adhesive bond or form fit or a combination of both. Therein, the components of a composite material can be composite materials themselves. In particle and fiber composite materials, particles or fibers are embedded in another component of the composite material, the so-called matrix. In fiber composite materials, the fibers can extend in one or more certain directions or have preferential directions. Fiber composite materials can be produced in layers, but are not yet layer composite materials thereby, if the consecutive layers are similar. However, the term laminate is also used here. Layer composite materials are constituted of layers of different number lying on top of each other. In penetration composite materials, the individual components each separately constitute contiguous open-pore materials. For example, they are produced by impregnating an open-pore sintered material (e.g. foamed ceramic) with a molten second substance.

In the present invention, all types of composite materials can be used for the installations, such as metal, ceramic or plastic, but all other commercially available materials are also conceivable. It is preferred that the antimicrobial active substance is incorporated in the composite material, and thus the composite material contains both one of the above mentioned materials and the antimicrobial active substance.

However, the use of so-called material combinations is also conceivable.

The material combination is also constituted of components of different materials with different characteristic profiles, which are combined to a component with new characteristic profile with the aid of a joining technology suitable for the material. In this combination, the individual used materials are mostly present in layers.

In the present invention, a material component of such a material combination can be the antimicrobial active substance itself, which is present as a layer in the combination. It then constitutes the material combination with a so-called substrate material, such as metal, ceramic and/or plastic.

Furthermore, it is preferred in the present invention that at least one component of the composite material and/or material combination includes a polymer matrix.

This polymer matrix can be composed of the commercially available polymers. Commercially available polymers are e.g. thermoplastic resins such as polyethylene and the copolymers thereof, polypropylene and corresponding copolymers, polyvinylchloride, polystyrene, polyester, polyether, polyamide, thermosetting resins such as phenolic resins, melamine resins, epoxy resins, rubber etc.

It is particularly preferred when the polymer matrix is composed of polyvinylchloride, polypropylene or a mixture thereof.

Furthermore, it is preferred when the used polypropylene is a polypropylene selected from the group including propylene homopolymer, copolymer of polypropylene and 0.1 to 25% by wt. ethylene, mixtures of propylene homopolymer or copolymer with HDPE, LDPE, LLDPE and/or EPR, mixtures of polypropylene with thermoplastic polymers such as polyamide, polyester. The proportion of the component mixed to the polypropylene should be between 0.1 and 40% by wt. in the mixtures.

Furthermore, it is preferred when the polypropylene is branched. Therein, a branched polypropylene as it is produced according to WO 00/00520 is preferred.

In addition, it is preferred when the polymer matrix additionally contains inorganic fillers such as talc besides the polymer.

In the present invention, it is preferred when the used droplet separator and/or the flow plates are made of a composite material, which contains or is composed of the above described polymer matrix and the antimicrobial active substance.

The antimicrobial active substance in turn contains molybdenum and/or tungsten.

While in the inorganic active agents available heretofore, the oligodynamic effect, i.e. the damaging effect of metal cations on living cells, is exploited, in the present invention, the formation of hydrogen cations causing decrease of the pH value in the medium being in contact with the substance is exploited. Therein, free protons immediately attach to a water molecule forming oxonium ions (H₃O⁺) due to their very small radius. If the concentration conditions allow, linkages of the oxonium ions to plural water molecules occur. Therefore, besides H⁺, the cations formed by reaction of H⁺ with water as well as the hydrates thereof are also referred to as hydrogen cations. Besides the oxonium ion (H₃O⁺), they are the Zundel cation (H_(S)O₂ ⁺) and the Eigen cation (H₉O₄ ⁺).

Thus, for example, molybdenum oxide reacts with water to molybdic acid (H₂MoO₄), which in turn reacts with H₂O to H₃O⁺ and MoO₄ ⁻ or MoO₄ ²⁻. Tungsten oxide also forms tungstic acid (H₂WO₄) with H₂O, which reacts with H₂O to H₃O⁺ and WO₄ ⁻ or WO₄ ²⁻. According to Arrhenius, the hydrogen cation is the carrier of the acidic properties. The pH value is the negative decadic logarithm of the numerical value of the hydrogen ion concentration in moles/liter. For a pure neutral solution of water, the hydrogen ions and the OH⁻ (hydroxide) ions have the same value (10⁻⁷ moles/l) and the pH value is 7. If a substance in contact with an aqueous medium forms hydrogen cations, thus, increase of the hydrogen cation value occurs, and therefore the aqueous medium becomes acidic.

Now, it has become apparent that substances forming hydrogen cations in contact with an aqueous medium have an excellent antimicrobial efficacy.

This effect already occurs in cooling towers by the air humidity contained in the cooling air. However, it can be increasingly observed in cooling tower systems working with cooling water.

Thus, the present invention is based on the idea according to the invention to completely prevent the formation of slime by non-elutable acidic surfaces. Additionally, it has become apparent that a temperature of 30 to 40° C. and an air humidity of 80 to 100% in the environment are advantageous for the efficacy of the used substance.

By formation of hydrogen cations, usually, decrease of the pH value of <6 occurs, a pH value of <5 is preferred.

Thus, a substantial advantage of the present invention is in that the antimicrobial substance is virtually not consumed. This is in particular the case if the substance has a low solubility in the aqueous medium as it is the case with molybdenum and/or tungsten or compounds thereof.

Therein, the solubility is preferably smaller than 0.1 moles/liter. The solubility of molybdenum and tungsten oxide is smaller than 0.02 moles/liter. Therefore, the antimicrobial effect is temporally almost unlimited present.

Thus, it is avoided that the installations containing the antimicrobial active substance have to be renewed due to the loss of the antimicrobial protection. The cooling towers according to the invention can be simpler serviced and maintained. Herein, additional cost will not arise. Furthermore, the antimicrobial active substance also does not enter the waters such that the present invention is harmless in terms of environmental protection regulations.

By the high effectiveness and the improved service friendliness, additional installations/constructions for cleaning and service become unnecessary. Therefore, the installations for the cooling system can also be packed more closely within the cooling tower, increasing the efficiency of the cooling tower with the same cooling volume.

Additionally, it has become apparent that it is preferred in the present invention when the molybdenum and tungsten containing materials are oxidized on the surface or are present in oxidic form.

In the characterization of the antimicrobial efficacy, it was resorted to a method, which is described in detail in the following scientific papers:

Fremdkörper-assoziierte Infektionen in der Intensivmedizin—Therapie und Prävention, J. P. Guggenbichler, Antibiotika Monitor 20 (3), 2004, p. 52-64

Inzidenz und Prävention Fremdkörper-assoziierter Infektionen, J. P. Guggenbichler, Biomaterialien 5 (4) 2004, p. 228-236.

In particular, the method of roll culture described there has well proven for investigating the antimicrobial effect. Therein, a sample of active agent is placed in a germ suspension for a certain time, for example 3 hours. Superficial growth of germs occurs. After this time, the samples are rolled over a so-called agar plate and placed in a sterile physiologic saline solution. This procedure is repeated multiple times every 3 hours. This repeated rolling operation in 3-hour distance gives information if and with which degree of efficiency a germ reducing or germ killing effect occurs. This method can be applied for the investigation of various bacteria such as Pseudomonas aeruginosa, Escherichia coli or Staphylococcus aureus.

The best results could be achieved with substances containing molybdenum and tungsten. Therein, it is preferred that molybdenum oxide or tungsten oxide develops in the border region between the molybdenum or tungsten based active agent.

In the used composite materials or material combinations, by thermal pre-oxidation at temperatures of advantageously greater than 300° C., an antimicrobial efficacy can be adjusted. The pre-oxidation can also be performed chemically or electrochemically. This pre-oxidation is required in solid Mo and W samples. Therein, it has become apparent that in comparison with an oxide film formed in situ, a material pre-oxidized by annealing is antimicrobially more effective. A pre-oxidation is to be performed in particular if the conditions of employment will not initiate a sufficient oxidation. Further, it is crucial that the oxide film has a large specific surface.

Besides pure molybdenum and pure tungsten, the compounds and alloys of these substances are also effective, which are sufficiently stable and form an oxide film on the surface. Among the molybdenum compounds having antimicrobial efficacy, besides the oxides, there are molybdenum carbide, molybdenum nitride, molybdenum silicide and molybdenum sulfide. The use of molybdenum oxide such as MO₂ or MO₃ is particularly preferred, the use of MO₃ is in particular preferred.

Molybdenum, molybdenum oxide and the above mentioned substances are also commercially available in very fine form with particle sizes according to Fisher of <1 μm. These alloys form an antimicrobial active oxide film on the surface.

In the case of tungsten, tungsten materials are also effective, which form an oxide film in situ or by precedent annealing. Besides oxidized pure tungsten, the oxides of the tungsten are effective. Here, in particular tungsten blue oxide (WO_(2.84)) and WO₃ are mentioned. Among the possible tungsten compounds forming an oxide film on the surface, tungsten carbide, tungsten silicide and tungsten sulfide are in particular suitable.

The described antimicrobial active substance can be mixed with the polymer used for the polymer matrix and then extruded to the corresponding installation together with the polymer to the polymer matrix or be present as a layer and constitute the composite material together with the polymer matrix.

Therein, it is preferred when the layer is partially oxidized. It is particularly preferred when the layer is composed of Mo oxide and/or W oxide, especially, it is preferred when it is composed of Mo and MO oxide or W and W oxide, it is in particular preferred when the layer is composed of MO₃.

This layer itself can be compact or porous. Particularly good results can be achieved if the layer has a spongiose, porous structure with a pore size of 50 to 900 μm. Such porous structures can for example be produced by depositing the antimicrobial active substance in the form of a slurry or from the gaseous phase with optional subsequent annealing. A large surface can also be achieved if the layer is present in the form of island-like, substantially non-contiguous agglomerates. It is particularly advantageous if these island-like agglomerates cover 40 to 90% of the surface of the substrate material.

As it has been shown, it is preferred when a very fine-grained powder is used, i.e. that the particle size should be <5 μm according to Fisher, smaller than 1 μm is more preferred.

Furthermore, it is preferred when the above described substance occurs in a proportion of 0.1 to 50% by vol. in the composite material and/or material combination, a proportion of 3 to 15% by vol. is in particular preferred.

Thus, the present invention provides a cooling tower, in which the risk of contamination with microorganisms, i.e. the deposition and proliferation thereof in the system, does no longer exist. The use of the described substance exhibits a particularly good cost-benefit ratio. Service and maintenance measures with respect to the antimicrobial protection are not necessary.

In the following, the particularly good antimicrobial efficacy of the substance is to be demonstrated once again based on TEM photographs and the stated test series.

FIG. 1 a-f: Various TEM photographs of a plastic sample, which contained MO₃ and was incubated for 6 h in 10⁹ CFU/ml Pseudomonas aeruginosa. Contamination of the sample with microorganisms could not be verified at any time.

FIG. 2: TEM photograph of a plastic sample, which did not contain any antimicrobial active substance and was incubated for 3 h with 10⁷ CFU/ml Pseudomonas aeruginosa. Clearly, a great infestation of the sample with Pseudomonas aeruginosa can be observed.

TEST SERIES

In the test series, the antimicrobial efficacy of MO₃ in comparison with copper was tested.

Therein, a composite material was used for the sample, which contained a polymer matrix of polypropylene.

For investigation of the antimicrobial efficacy, the already described roll culture was used.

The sample was incubated for 6 h in 10⁹ CFU/ml Pseudomonas aeruginosa.

The incubation temperature was 37° C. and there was an air humidity of ca. 100%, which corresponds to conditions as they occur in cooling towers.

In the “copper samples”, first, the growth and the proliferation of Pseudomonas aeruginosa was prevented, which was to be expected. However, the antimicrobial efficacy of the samples decreased after some hours and increase of the number of germs occurred. In addition, on the surface of the used composite material, the formation of copper sulfate (CuSO₄) could be verified. This phenomenon can only be explained in that the present copper in the samples is consumed in the course of time and thus the antimicrobial effect is lost.

The TEM photographs depicted in FIG. 1 a-f show that the antimicrobial effect in the samples containing WO₃ is permanently present. Furthermore, also decrease of the effect as it was the case in the “copper samples” was not observed.

Thus, it could be proven that the substance according to the invention exhibits a very good antimicrobial effect under conditions as they exist in cooling towers, and moreover, it is temporally unlimited. 

1. Cooling tower including a cooling system, wherein the cooling system has at least one installation of at least one composite material and/or at least one material combination and a substance causing the formation of hydrogen cations in contact with an aqueous medium and containing molybdenum and/or tungsten.
 2. Cooling tower according to claim 1, wherein the cooling system is a cooling water system.
 3. Cooling tower according to claim 2, wherein the installation is a droplet separator.
 4. Cooling tower according to claim 2, wherein the installation is a flow installation.
 5. Cooling tower according to claim 1, wherein at least one component of the composite material and/or of the material combination contains a polymer matrix.
 6. Cooling tower according to claim 5, wherein the polymer matrix contains polypropylene.
 7. Cooling tower according to claim 1, wherein the substance is incorporated in the polymer matrix.
 8. Cooling tower according to claim 2, wherein the solubility of the substance in the cooling water is smaller than 0.1 moles/liter.
 9. Cooling tower according to claim 1, wherein the substance is Mo oxide or W oxide.
 10. Cooling tower according to claim 1, wherein the Mo oxide is MoO₂ and/or MoO₃.
 11. Cooling tower according to claim 1, wherein the W oxide is tungsten blue oxide and/or WO₃.
 12. Cooling tower according to claim 1, wherein the substance is molybdenum, a molybdenum alloy and/or a molybdenum compound, wherein the surface has a Mo oxide layer.
 13. Cooling tower according to claim 1, wherein the substance is tungsten, a tungsten alloy and/or a tungsten compound, wherein the surface has a tungsten oxide layer.
 14. Cooling tower according to claim 12, wherein the molybdenum compound is molybdenum carbide, molybdenum nitride, molybdenum silicide and/or molybdenum sulfide.
 15. Cooling tower according to claim 13, wherein the tungsten compound is tungsten carbide, tungsten nitride, tungsten silicide and/or tungsten sulfide.
 16. Cooling tower according to claim 1, wherein the substance is present as a component of a layer.
 17. Cooling tower according to claim 16, wherein the layer is composed of Mo oxide and/or W oxide.
 18. Cooling tower according to claim 16, wherein the layer is composed of Mo and Mo oxide or W and W oxide.
 19. Cooling tower according to claim 16, wherein the layer is composed of MoO₃.
 20. Cooling tower according to claim 1, wherein the substance has a particle size according to Fisher of <5 μm.
 21. Cooling tower according to claim 7, wherein the mass content of the substance in the composite material and/or material combination is 0.1 to 50% by vol. 